Theatre Design & Technology - Oct 1973 - 14

reverberation time as a measure of the suitability of
speech acoustics in a given hall or theatre. The difficulty
with these more sophisticated criteria for speech acoustics
is that they are nearly impossible to calculate from twodimensional drawings of a hall or theatre and require
fairly involved equipment for measurement in the completed theatre or in a scale model.
Reverberation time is both simpler to calculate from
drawings and measure in a completed theatre. However,
there is clearly no single number, no specific reverberation
time that all theatres should have for optimum results.
Successful theatres (acoustically) have been built with
measured mid-frequency reverberation times between 0.3
seconds and 1.7 seconds, all having good speech acoustics
without electronic reinforcement. In general, the lower
reverberation times, under 1.0 seconds, will be found
more applicable to arena-stage and thrust-stage theatres
where long-delayed reflections must be minimized because
there may be very little direct and early reflected energy
to start with (when the actor's back is to the listener); and
the longer reverberation times applicable to open-stage
and proscenium unidirectional fan-plan and rectangular
theatres, particularly with hard "box" sets that efficiently
reinforce an actor's voice. Reverberation time calculations
should be a check on overall design, today, rather than the
basis of design.
One is left, then, with the problem of what to do with
the wall and ceiling surfaces that produce long-delayed,
rather than short-delayed reflections. Often these are the
backs of light covers and rear walls. Much depends on the
answer to the question: Will a specific surface, if sound
reflecting, merely add to reverberation, or cause a specific
acoustical problem, such as echo (a discreet long-delayed
reflection heard by the I istener as a second signal) or
flutter (a rapidly repeating echo)? A surface that can cause
an acoustical problem clearly must be reshaped to avoid
that problem or be treated with efficient sou nd-absorbing
material.
Most rear wall surfaces will, if sound-reflecting, return
long-delayed sound energy to the stage and to the front
seats of the audience. If this energy creates a loud echo
there will clearly be a problem; but if it returned as
moderate reverberant energy, it can assist the actor in
letting him know that the house is "live," and that he is
"getting across" to the audience. Ray diagram studies during the design stages clearly showed that the large, continuous, concave rear wall surfaces at the Alley Theatre,
Houston, and the Vivian Beaumont, New York, would
create echo problems, and these rear walls are treated with
efficient sound-absorbing material. The treatment on the
rear wall of the Forum Theatre (the Beaumont's "little"
theatre) was unfortunately left off at the opening; it was
rapidly found needed and was added. On the other hand,
the lower rear wall (under the balcony) of the Fisher
Theatre in Detroit and the entire rear wall area of the

Walnut Street Theatre in Philadelphia are hard and soundreflecting and return some sound energy to the stage. The
distinction is that these sound-reflecting rear wall surfaces
have considerable large-scale surface modulation or "breakup" and scatter sound energy, instead of returning it at
one time and to one place. Also, these theatres are not
basically spaces where the "behind-the-actor's back" situation occurs, and reverberant energy is, therefore, less
harmful.
Paper ray-diagram studies are helpful in locating potential echoes and flutter effects; light-model studies using
silvered paper for sound-reflecting surfaces in relatively
small scale models can be even more useful, but the most
useful technique of all in evaluating such problems is the
acoustical model study, using relatively large scale models
(1/10-1/25 actual size). impulsive sound sources (a spark
gap), and small microphones. An impulse response properly presented on an oscilloscope can yield a great deal of
information on the acoustics of a theatre, including data
for calculation of the articulation index, and the acoustical model study should be a recommended procedure for
any unusually designed theatre that is not based on
experience with past examples.
When good judgment or ray diagram or model tests
indicates that an interior wall surface or, most unusual,
a ceiling surface should be made sound-absorbing, there
are many materials to choose from. Glass fiber over an airspace and behind expanded metal, wire cloth, and/or grill
cloth, glass fiber beh ind carpet, carpet over an air space,
sound-absorbing form board (such as Tectum or Insulrock),
furred acoustic tile, these and many more each have their
particular combination of architectural and acoustical
properties for suitability as the specific sound-absorbing
treatments for specific problems. Heavy velour or cottonflannel drapes may be the best "fix" in existing theatresor new theatres.
I n any well-designed theatre, however, the main soundabsorbing element will be the audience itself. I ndeed, the
greater predictability of acoustical results in theatres and
concert halls over the past twenty years has partly been
due to greater accuracy in estimating the sound absorption of audiences, now usually predicted on a per-unitarea, rather than a per-seat basis. And the acoustics of any
theatre should be designed for optimum results when
occupied. A frequent error in the distant past was the
application of large areas of sound-absorbing material to
"reduce the reverberation" of an empty theatre, only to
have it prove far too dead when occupied; let us avoid th is
mistake today.

Seats
If we design for full occupancy conditions, what about
rehearsals and partial occupancy? The best answer lies in
the seat upholstery. Porous fabric cushion covering over
fill ing materials with interconnecting pores can result in a
seat that largely compensates for the sound absorption of

THEATRE DESIGN AND TECHNOLOGY

OCTOBER. 1973

Table of Contents for the Digital Edition of Theatre Design & Technology - Oct 1973